1. Field of the Invention
The present invention relates to an apparatus for inspecting defects and foreign substances, more specifically, to an apparatus which can be preferably used for inspecting defects in circuits on a reticle or a photomask used as an original plate in manufacturing a semiconductor element, and the like, or for inspecting defects, including foreign substances, on a substrate such as a semiconductor wafer.
2. Related Background Art
For example, in order to inspect defects in circuit patterns on a reticle or a photomask which is used as an original plate in manufacturing a semiconductor element, or in order to inspect foreign substances on a substrate such as a semiconductor wafer, an apparatus called "defect and foreign substance inspecting apparatus" has been used.
FIG. 5A shows such a typical defects and foreign substance inspecting apparatus, which is used for, for example, inspecting defects (including foreign substances) which are present in the periodical structure of a semiconductor wafer on which many circuit patterns are formed. FIG. 5C shows the semiconductor wafer 1 which is the object to be inspected. Generally, the semiconductor wafer 1 has a regular array of circuit units (hereinafter referred as "dies") 2, each of which has at least several tens of circuit patterns along both the X axis and the Y axis. A typical die 2 is a square of ca. 20 mm.times.20 mm.
The above-mentioned defect and foreign substance inspecting apparatus is disclosed in U.S. Ser. No. 060090, filed on Jun. 8, 1987.
As shown in FIG. 5A, the inspecting apparatus has a laser light source 3. A monochromatic laser beam emitted from the laser light source 3 is converted into a substantially collimated beam 5 having a predetermined diameter by a beam expander 4. The beam 5 is focused at a focal point 7 in a rear focal surface of a lens 6 by the lens 6. A beam 8 diverging from the focal point 7 is reflected by a reflecting mirror 9 arranged in the vicinity of the focal point 7. The beam reflected by the reflecting mirror 9, which has a circular cross section, travels towards a Fourier transform lens 10.
The effective center of the lens 10 is arranged to be at a position a little nearer than the focal distance of the lens 10. A collimated beam 11 emitted from the lens 10 is incident on the surface of the semiconductor wafer on which the patterns are formed. The semiconductor wafer 1 is held in a chuck 13 which is a part of a two-dimensional parallel displacement means 12. The two-dimensional parallel displacement means 12 can displace the semiconductor wafer 2 two-dimensionally in a plane vertical to the optical axis of the lens 10. The semiconductor wafer 1 is arranged in an object surface (that is, the front focal surface) of the lens 10 so that the surface of the semiconductor wafer 1 on which the patterns are formed is illuminated with the collimated beam 11.
As shown in FIG. 5B, an illuminated area 15 having a diameter of 20 mm in the surface of the semiconductor wafer 1 is illuminated with the collimated beam 11. As shown in FIG. 5A, a beam 16 diffracted by the illuminated area of the semiconductor wafer 1 is led through the lens 10 to a Fourier transform plane (that is, the rear focal surface) 17 of the lens 10, on which the image of the Fourier transform pattern corresponding to the circuit patterns in the illuminated area on the surface of the semiconductor wafer 1 is formed.
Incidentally, if the reflecting mirror 9 employed in the optical system shown in FIG. 5A is a half mirror, and if the semiconductor wafer 1 does not have any circuit patterns, that is, no circuit patterns have not yet been formed, the constitution of the apparatus in the vicinity of the reflecting mirror 9 shown in FIG. 5A is replaced by a corresponding constitution shown in FIG. 6. In the inspecting apparatus shown in FIG. 6, the diameter of the beam spot focussed at the rear focal point 7 of the lens 6 is in inverse proportion to the diameter of the monochromatic collimated beam 5 emitted from the beam expander 4 towards the lens 6. The beam 8 diverging from the focal point 7 travels towards a half mirror 18, and the beam, which has a circular cross section, reflected by the half mirror 8 travels towards the lens 10. The collimated beam 11 emitted from the lens 10 is reflected by a wafer 1A having no circuit pattern, is transmitted through the lens 10 again to be a beam 19, and the beam 19 forms a beam spot 20 on the Fourier transform plane 17 of the lens 10. The diameter of the beam spot 20 is substantially the same as that of the beam at the focal point 7.
To return to FIGS. 5A and 5C, the illuminated area 15 on the semiconductor wafer 1 having the diameter of 20 mm gives sufficiently exact Fourier transform patterns. For, the semiconductor wafer 1 has a lot of circuit patterns.
A previously prepared spatial filter 21 is provided in the Fourier transform plane (the rear focal surface) 17 of the Fourier transform lens 10. The spatial filter 21 can be prepared by exposing a recording medium such as a photographic dry plate to the light diffracted by all the dies on the semiconductor wafer 1. The semiconductor wafer 1 used here may be the very semiconductor wafer to be inspected even if some patterns of said semiconductor wafer 1 have defects. For only the Fourier transform beams informing the normal patterns of the semiconductor wafer 1 can expose the photographic dry plate with relatively high intensity, while the relatively weak beams informing the defects can not.
Therefore, the spatial filter 21 blocks the spatial frequency of the non-defective Fourier transform information of the illuminated dies 2 on the semiconductor wafer 1, but transmits the beams generated by the defects in these dies 2. A beam 22 conveying the defect information which is not blocked by the spatial filter 21 is incident on an inverse Fourier transform lens 23. Though the lens 23 is illustrated as a single lens in the drawing, the lens 23 may consists of a plurality of lens elements. The lens 23 is arranged at a position away from the Fourier transform plane 17 of the lens 10, wherein the distance between the lens 23 and the Fourier transform plane 17 is the same as the focal distance of the lens 23. The lenses 10 and 23 are aligned along the same optical axis 24. The two-dimensional parallel displacement means 12 displaces the semiconductor wafer 1 in the direction vertical to the optical axis 24. The lens 23 performs the inverse Fourier transform of the filtered light patterns of the illuminated dies 2 on the semiconductor wafer 1 and forms the images of the defective dies 2 in the rear focal surface, that is, the image surface 25 of the lens 23.
A photo detector array 26 is arranged in the rear focal surface of the lens 23, that is, on the image surface 25 on which the images of the defects are formed so that the center of the photo detector array 25 coincides with the optical axis 24. Each light-receiving element of the photo detector array 26 receives images of the defects present in the dies 2 on the optical axis 24.
According to the above-mentioned conventional art, the Fourier transform lens 10 and the inverse Fourier transform lens 23 should be designed on highly exacting conditions in order to sufficiently reduce electronic or optical noises.
In order to sufficiently reduce optical noises, the minimum spot diameter d1 on the Fourier transform plane 17 and the minimum spot diameter d2 on the image surface are strictly limited. On the other hand, in order to sufficiently reduce electronic noises, the lenses 10 and 23 have to be able to collimate the beams diffracted into space defined as a circular cone which is telecentric more than .+-.15.degree. from any positions in the field of view in the object surface (the front focal surface) 14 of the lens 10 with sufficiently little aberration, and have to finally form images of paraxial diffracted beams with very little geometrical distortions.
In order to satisfy the above-mentioned condition, as shown in FIGS. 5B and 5C, the observable field 27 in which the defects can be inspected with sufficiently little electronic or optical noises should be smaller than the illuminated area 15 on the semiconductor wafer 1. The size of the illuminated area is determined so as to realize the optimal exposure of the spatial filter 21, and the entire illuminated area 15 must be illuminated even when only the observable field 27 is observed. For the Fourier transform patterns of the circuit patterns in the illuminated dies 2 at the time of the preparation of the spatial filter 21 needs to be the same as the Fourier transform patterns at the time of the defect inspection operation.
In order to explain the above situation, suppose an inspecting apparatus which illuminates as large an area as the observable field 27 shown in FIG. 5B. That is, as shown in FIG. 7, a collimated beam 29 with a small diameter which is emitted from the Fourier transform lens 10 illuminates only the observable field. In this case, the non-observable reagion in the illuminated area 15 which is indicated by slant lines in FIG. 5B does not have to be illuminated. But, as shown in FIG. 7, the diameter of a beam spot 31 formed in the Fourier transform plane 17 by a reflected beam 30 from the wafer 1A without circuit patterns is greater than the diameter of the beam spot 20 formed in FIG. 6, so the beam spot 31 can not be completely shielded by the spatial filter in the Fourier transform plane 17. Therefore, the entire illuminated area 15 has to be illuminated also at the time of the defect inspection operation, which makes it difficult to illuminate the observable field 27 with illumination light having sufficient illuminance.
In addition, the illuminance of the illumination light with which the observable field 27 shown in FIG. 5C is illuminated affects the time required for inspection. Since the observable field is not large enough to observe a whole die 2 on the semiconductor wafer 1 at the same time, the observable field 27 should be shifted within the die 2. The velocity of relative displacement of the observable field 27 with respect to the die 2 is determined according to the light cumulative time of the photo detector array 26 and the illuminance of the light with which the observable field is illuminated, wherein the lower the illuminance becomes the smaller the velocity of relative displacement will be, of course. Incidentally, when a light cumulative method such as time delay integration (TDI) is employed by the photo detector array 26, as is sometimes the case, highly exact operation of relative displacement is required.
In order to sufficiently reduce electronic or optical noises according to the above-mentioned conventional art, the Fourier transform lens 10 and the inverse Fourier transform lens 23 should be designed on highly exacting conditions. In addition, it is difficult to illuminate the observable field 27 with illumination light having sufficient illuminance. Therefore, according to the conventional art, charge storage photo detector elements are used in the photo detector array 26 and technique such as the time delay integration (TDI) is employed which synchronize the relative displacement of the observable field 27 on the semiconductor wafer 1 to be inspected with charge storage in the photo detector array 26 and charge shift within picture elements. When such technique is employed, however, highly exact operation of relative displacement of the object to be inspected is required, which is very difficult.
The above difficulties rise because the substantially collimated beam is used as the illumination light with which the object to be inspected is illuminated.